The common name is sure Poisoned Glen, in Bearla, on the other hand the Irish name is Cro Nimhe. Cro = enclosure, pen. Nimhe/Neimhe is apparently poison. I found these explanations on Google:

"How can a place so beautiful be called "poisoned?" Located in Donegal near the Glenveagh National Park on the road between Bunbeg and Letterkenny, the Poisoned Glen with its quartzite mountainsides looks and sounds forboding. One explanation for its name is that a type of plant, Irish spurge, grows in the glen and is poisonous. There is also a legend that the stone seen at the entrance of the Glen, known as "the Evil Eye of Balor," is poisonous. However, the exact origins of the name may be attributed to an error made by an English cartographer (it figures). The Glen was once known in Gaelic as the "Heavenly Glen," but the cartographer misspelled the Gaelic word for Heaven which is "Neamh" with the word "Neimhe" which is Gaelic for poison. Thus, the name stuck as "The Poisoned Glen.""

"But the explanation favored by most people, tells the story of Balor, a king on Tory Island, and his beautiful daughter. So beautiful in fact that Balor felt compelled to imprison her in a tower so that she would not come within sight of men. But the fame of her looks spread and men came from far and wide to attempt to see and woo the beautiful princess. Eventually, one gallant succeeded in capturing and spiriting her back to the mainland, up into the mountain fastness around Errigal. Balor followed the pair across the sea and up the valley into the Poisoned Glen. There he killed the captor with a giant stone that now stands at the entrance to the Glen. That stone is said to be the evil, or "poisoned" eye of Balor. And hence the Poisoned Glen."

Surely the term is ancient, so that the word "poisoned" might not necessarily refer to chemicals. The idea it might represent "evil" perhaps is nearer the mark: poisoned in a spiritual sense. Spiritually poisoned implies the presence of evil spirits, hence "haunted." Mountains generally at one time were thought to be the abode of evil spirits.

Apparently a similar word, or the same word, occurs in Ben Nevis. Irish 'mh' is often pronounced as if it were 'v.' If it's not 'v' it's 'w' (never mh).